Clivia Hejny

Polysomatism in högbomite: The crystal structures of 10T, 12H, 14T, and 24R polysomes

Abstract Högbomite is a closest-packed polysomatic mineral composed of spinel, T2M4O8, and nolanitelike, TM4O7(OH), modules where T stands for tetrahedrally and M for octahedrally coordinated cations. The modules are stacked in an ordered fashion in various ratios. Single-crystal X-ray diffraction for a 24R and a 10T polysome and structure modeling for a 12H and 14T polysome have been applied to characterize different stacking variants. Högbomite from a spinel-phlogopite schist at Corundum Creek (South Australia) with composition Mg3.8Fe3.2Zn1.6Ti1.0 Al18.3O38 (OH)2 is a 10T polysome with a = 5.723(1), c = 23.026(4) Å, space group P3̄m1, Z = 1. This polysome with the general formula T8M20O38(OH)2 is composed of an alternation of spinel (S) and nolanite-like (N) blocks stacked in the sequence NSSNS. Högbomite from a Fe-Ti deposit at Liganga (Tanzania) with composition Mg13.5Fe5.6(Zn,Mn,Ni)0.2 Ti4.7Al41.7(Cr,Ga)0.2O90(OH)6 is a 24R polysome with a = 5.7145(7), c = 55.056(5) Å, space group R3̄m, Z = 1. The structure with the general formula T18M48O90(OH)6 is composed of a periodic alternation of two S and two N blocks. The crystal structures of högbomite-12H, T10M24O46(OH)2, a = 5.7, c = 27.6 Å, space group P63mc, Z = 1, and högbomite-14T, T12M28O54(OH)2, a = 5.7, c = 32.2 Å, space group P3̄m1, Z = 1, were modeled from the stacking principles of the known 6T, 8H, 10T, and 16H polysomes. The 12H and the 14T polysomes have stacking sequences NSSNSS and NSSSNSS, respectively.

Enhanced kinetic stability of pure and Y-doped tetragonal ZrO2.

The kinetic stability of pure and yttrium-doped tetragonal zirconia (ZrO2) polymorphs prepared via a pathway involving decomposition of pure zirconium and zirconium + yttrium isopropoxide is reported. Following this preparation routine, high surface area, pure, and structurally stable polymorphic modifications of pure and Y-doped tetragonal zirconia are obtained in a fast and reproducible way. Combined analytical high-resolution in situ transmission electron microscopy, high-temperature X-ray diffraction, and chemical and thermogravimetric analyses reveals that the thermal stability of the pure tetragonal ZrO2 structure is very much dominated by kinetic effects. Tetragonal ZrO2 crystallizes at 400 °C from an amorphous ZrO2 precursor state and persists in the further substantial transformation into the thermodynamically more stable monoclinic modification at higher temperatures at fast heating rates. Lower heating rates favor the formation of an increasing amount of monoclinic phase in the product mixture, especially in the temperature region near 600 °C and during/after recooling. If the heat treatment is restricted to 400 °C even under moist conditions, the tetragonal phase is permanently stable, regardless of the heating or cooling rate and, as such, can be used as pure catalyst support. In contrast, the corresponding Y-doped tetragonal ZrO2 phase retains its structure independent of the heating or cooling rate or reaction environment. Pure tetragonal ZrO2 can now be obtained in a structurally stable form, allowing its structural, chemical, or catalytic characterization without in-parallel triggering of unwanted phase transformations, at least if the annealing or reaction temperature is restricted to T ≤ 400 °C.

Looking for jarosite on Mars: The low-temperature crystal structure of jarosite

Abstract Single-crystal diffraction of jarosite, KFe33+(SO4)2(OH)6, has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3̅m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO12 polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X‑ray diffraction of martian soils using the Curiosity Rover’s CheMin instrument.

High-Temperature Carbon Deposition on Oxide Surfaces by CO Disproportionation

Carbon deposition due to the inverse Boudouard reaction (2CO → CO2 + C) has been studied on yttria-stabilized zirconia (YSZ), Y2O3, and ZrO2 in comparison to CH4 by a variety of different chemical, structural, and spectroscopic characterization techniques, including electrochemical impedance spectroscopy (EIS), Fourier-transform infrared (FT-IR) spectroscopy and imaging, Raman spectroscopy, and electron microscopy. Consentaneously, all experimental methods prove the formation of a more or less conducting carbon layer (depending on the used oxide) of disordered nanocrystalline graphite covering the individual grains of the respective pure oxides after treatment in flowing CO at temperatures above ∼1023 K. All measurements show that during carbon deposition, a more or less substantial surface reduction of the oxides takes place. These results, therefore, reveal that the studied pure oxides can act as efficient nonmetallic substrates for CO-induced growth of highly distorted graphitic carbon with possible important technological implications especially with respect to treatment in pure CO or CO-rich syngas mixtures. Compared to CH4, more carbon is generally deposited in CO under otherwise similar experimental conditions. Although Raman and electron microscopy measurements do not show substantial differences in the structure of the deposited carbon layers, in particular, electrochemical impedance measurements reveal major differences in the dynamic growth process of the carbon layer, eventually leading to less percolated islands and suppressed metallic conductivity in comparison to CH4-induced graphite.

High pressure orthorhombic structure of CuInSe2

The structural behaviour of CuInSe(2) under high pressure has been studied up to 53 GPa using angle-dispersive x-ray powder diffraction techniques. The previously reported structural phase transition from its ambient pressure tetragonal structure to a high pressure phase with a NaCl-like cubic structure at 7.6 GPa has been confirmed. On further compression, another structural phase transition is observed at 39 GPa. A full structural study of this high pressure phase has been carried out and the high pressure structure has been identified as orthorhombic with space group Cmcm and lattice parameters a = 4.867(8) Å, b = 5.023(8) Å and c = 4.980(3) Å at 53.2(2) GPa. This phase transition behaviour is similar to those of analogous binary and trinary semiconductors, where the orthorhombic Cmcm structure can also be viewed as a distortion of the cubic NaCl-type structure.

Description and crystal structure of turtmannite, a new mineral with a 68 Å period related to mcgovernite

Abstract Jacobsite-rich Fe-Mn ores of probable Dogger age fill paleokarst pockets in the Triassic marbles of the Barrhorn Unit under Pipjigletscher in the Turtmanntal, Valais, Switzerland. These ores and embedding rocks underwent Tertiary metamorphism under upper greenschist facies conditions. Some of these jacobsite ores contain minor amounts of a yellow micaceous mineral, which appears to be a new Mn-Mg silicate-vanadate-arsenate that was named “turtmannite” with respect to the type locality. Turtmannite flakes up to 200 μm in length occur parallel to the main schistosity, or fill thin discordant veinlets. Turtmannite is rhombohedral R3̄c, with aH = 8.259(2) and cH = 204.3(3) Å in the hexagonal setting. The corresponding primitive rhombohedral cell has aR = 68.31 and αR = 6.92°. HRTEM images indicate that turtmannite is perfectly ordered along c. The structure of turtmannite has been solved to a final R1 of 12.4% on a Siemens Smart CCD diffractometer with MoKα X-radiation, and a detector to sample distance extended to 12 cm. The structure consists of 84 oxygen layers stacked along c, with twelve close-packed layers followed by two non-close-packed layers. This sequence is repeated six times. The structure contains eight symmetrically distinct cation layers. Three different occupational variants have been recognized leading to the following hypothetical end-member formulae and approximate abundances: I [IV]Mn1.5[IV]Mg3[VI](Mn,Mg)21[(V,As)O4]3[SiO4]3O5(OH)20, 50% II [IV]Mn1.5[VI](Mn,Mg)21[(V,As)O4]3[SiO4]3[AsO3](OH)21 33% III [IV]Mn1.5[VI](Mn,Mg)21[(V,As)O4]3 [SiO4]2[SiO3OH](OH)25 16% The simplified chemical formula for turtmannite can be written as: (Mn,Mg)22.5Mg3-3x[(V,As)O4]3[SiO4]3[AsO3]xO5-5x(OH)20+x. The unit cell of turtmannite is similar to those of mcgovernite, a Mn-Mg-Zn arsenate from Sterling Hill, New Jersey, and an unnamed “mcgovernite-like” Mn-Mg arsenate from the Kombat Mine, Namibia. The crystal structure of turtmannite is close to the model predicted for mcgovernite by Moore and Araki (1978)

Second-order P6̄c2-P31c transition and structural crystallography of the cyclosilicate benitoite, BaTiSi3O9, at high pressure

Abstract Experimental high-pressure investigations on benitoite in the diamond-anvil cell reveal a secondorder phase transition at a critical transition pressure Pc = 4.24(3) GPa, as determined from synchrotron powder diffraction, single-crystal X-ray diffraction, and Raman spectroscopy. Diffraction experiments indicate a non-isomorphous transition from P6̄c2 to P31c space-group symmetry with a′ = a√3 and c′ = c relative to the P6̄2c subcell below Pc. The high-pressure polymorph is characterized by a larger compressibility compared to the compressional behavior of benitoite below Pc. Fitting second-order Birch-Murnaghan equations of state to the experimental data sets, the parameters obtained are V0 = 372.34(4) Å3, K0 = 117.9(7) GPa, with a0 = 6.6387(3) Å, Ka = 108.1(7) GPa, and c0 = 9.7554(4) Å, Kc = 143.3(1.1) GPa for the low-pressure form (P < Pc), and V0 = 376.1(4) Å3, K0 = 88.9(1.6) GPa, with a0 = 11.516(4) Å, Ka = 95.4(1.8) GPa, and c0 = 9.826(4) Å, Kc = 77.2(1.6) GPa for the high-pressure form (P > Pc). One of the most significant structural changes is related to the coordination of Ba atoms, changing from an irregular [6+6] coordination to a more regular ninefold. Simultaneously, the Si3O9 rings are distorted due to no longer being constrained by mirror-plane symmetry, and the Si atoms occupy three independent sites. The higher compressibility along the c-axis direction is explained by the relative displacement of the Ba position to the Si3O9 rings, which is coupled to the lateral displacement of the non-bridging O2-type atoms of the ring unit. A symmetry mode analysis revealed that the transition is induced by the onset of a primary order parameter transforming according to the K6 irreducible representation of P6̄c2.

Crystal chemistry of the polysome ferrohögbomite-2N2S, a long-known but newly defined mineral species

Ferrohogbomite-2 N 2 S is a polysomatic hogbomite-group mineral composed of two nolanite modules ( N ) and two spinel modules ( S ) where the spinel modules have ideally hercynite composition. Ferrohogbomite has been described in the literature for a long time but was not distinguished from magnesiohogbomite and simply named hogbomite. The revised nomenclature of hogbomite minerals required definition of ferrohogbomite polysomes as new mineral species. The sample from Ain Taiba, northwestern edge of Grand Erg Oriental in the Algerian Sahara, has the simplified composition IV (Fe 2+ 3 ZnMgAl) Σ = 6 VI (Al 14 Fe 3+ Ti 4+ ) Σ = 16 O 30 (OH) 2 . The crystal structure was refined from single-crystal X-ray data. The mineral crystallizes in the acentric space group P 6 3 mc with a = 5.712(1), c = 18.317(7) A, and shows twinning by merohedry with the inversion centre as twinning operation. This ferrohogbomite-2 N 2 S is associated with ilmenite, hematite, minor magnetite, and pseudorutile. Some of the ferrohogbomite-2 N 2 S grains are rimmed by hercynite, Fe 0.57 Zn 0.26 Mg 0.18 Al 2 O 4 , which separates them from hematite. The original assemblage probably consisted of ilmenite and Al-bearing magnetite or another Ti-bearing spinel phase. During oxidation ilmenite was replaced by ferrohogbomite on its rims and magnetite was oxidized to hematite. Finally, residual ilmenite was partly transformed along cracks and grain boundaries to pseudorutile. A HRTEM study showed homogeneous ferrohogbomite-2 N 2 S with minor planar faults increasing towards the grain boundaries. The observed orientation relation between ferrohogbomite-2 N 2 S and hematite suggests that ferrohogbomite-2 N 2 S formed topotactically from a primary spinel or magnetite phase that had been subsequently oxidized to hematite.

NEW RESULTS ON OLD PROBLEMS: THE USE OF SINGLE-CRYSTALS IN HIGH-PRESSURE STRUCTURAL STUDIES

Single-crystal X-ray diffraction techniques have been applied to a number of outstanding structural problems in elemental metals. Cs-III, Rb-III and Ga-II are shown to be members of a new class of modulated elemental structure containing different stackings of 8- and 10-atom layers; the interaction of the host and guest components in the incommensurate composite structure of Bi-III is found to induce modulations of the basic host and guest structures; and Te-III is shown to have a body-centred monoclinic structure modulated by an incommensurate displacement wave.

High-pressure crystallography of periodic and aperiodic crystals

This article discusses the high-pressure behaviour of molecular crystal structures, energetic materials, phases relevant to the Earth’s interior, materials with a pressure-induced expansion in one or two directions, dealing with high-pressure data from crystals with twinning and pseudosymmetry, pressure-induced phase transitions including an incommensurate phase, and technical developments.